Why are some cakes one and other cakes the other? Is it ingrediants, baking, or a combination?
A combination. I prefer denser cakes myself.
Add mixing technique in there as a factor, too. Lots of cakes that are supposed to be light come out heavy and dense in the wrong hands
Does using butter vs. oil play a role?
Oil cakes can tend to be heavier than butter cakes, more like a muffin if that makes sense. On the other hand, chiffon cakes are oil- based, so that isn't always true.